The present invention relates to the alignment of a source of an optical signal (for example an end of an optical fibre) with an optical waveguide (for example another optical fibre). The invention is particularly suited to alignment of monomode fibres prior to splicing, and requires neither access to a remote free end nor a transparent buffer or jacket of one of the waveguides.
Many methods have been proposed in the prior art for aligning fibres. The simplest methods, perhaps, are mere mechanical methods where alignment between the outer surfaces of the fibres, generally of the claddings since the buffers and other outer layers will be removed for alignment, is achieved by laying the fibres in a precisely machined V-groove or similar structure. The invention, of course, is to align the fibre cores, and this method works tolerably well for multi-mode fibres, which have cores of diameter say 50 microns within a cladding of diameter say 125 microns, since at these dimensions any core eccentricity is unlikely to be a significant problem.
The situation with monomode fibres is quite different. Here the core diameter is much smaller, typically 5-10 microns, and any core eccentricity can easily be sufficiently great with respect to the core size that alignment of the claddings allows no transmission of core modes.
That problem has in general been solved by optical, rather than mere mechanical, methods of alignment; an optical signal is directed into the core of one fibre and withdrawn from the other fibre, the efficiency of transmission from one fibre to the other giving an indication of the accuracy of alignment. When the fibres are properly aligned they are spliced in that position, for example by fusing or bonding them together.
One such optical method comprises injecting light into the core at one remote end of one of the fibres to be aligned, and monitoring the intensity of light emitted from the remote end of the other fibre. The ends to be joined are moved relative to one another until the emitted intensity reaches a maximum. A modification of this basic idea is disclosed in DE No. 2,626,839 (Siemens) where it is suggested that instead of seeking the maximum intensity of light emitted at the end of the other fibre, one can seek the minimum amount of light scattered away from the fibres at the fibre junction. The assumption, presumably, is that if light is not scattered away from the fibres and into detectors placed around the junction, then it is being transmitted into the other fibre. Whilst this modification has the advantage of not required access to a remote end of the other fibre (which would be inconvenient because it may be a large distance away from the splice position), it is not clear that a lack of such scattering can be directly correlated to successful transmission of light along the other fibre. For example generation of cladding modes, rather than the desired core modes, in the other fibre would appear also to be at the expense of light scattered at the junction.
Access to a free end of a fibre can also be avoided by injecting light into and withdrawing light from respective fibres at positions adjacent the ends to be spliced. An excellent method for doing this is described and claimed in GB No. 2,100,463B. The fibre is bent at some point near the splice to be made and an optical coupler is placed adjacent the fibre buffer, allowing an optical signal to be injected into or withdrawn from the fibre core via the fibre buffer. Whilst this method gives good results it is clearly not applicable to cabled fibres or other fibres having opaque buffers or jackets.
We have now devised a method of aligning, which does not need (although it may make use of) a remote end of a fibre or a transparent fibre outer layer. The invention is based on the different backscattering performance of core modes, cladding modes and buffer modes etc. Backscattering of light will occur at optical interfaces, and in addition there will be a continuum of reflected energy by a phenomenon called Raleigh backscattering.